Synergism in sequential disinfection of Cryptosporidium parvum

Sequential treatment schemes using ozone or chlorine dioxide as primary dis infectant and free chlorine or monochloramine as secondary disinfectant wer e investigated for potential synergistic effects in the inactivation of C. parvum oocysts. Viability was determined by a modified in vitro technique b ased on sporozoite counts that was consistent with animal infectivity data. Primary disinfection with ozone resulted in the disappearance of a predomi nant shoulder in the secondary inactivation curve for both free and combine d chlorine. The rate of secondary inactivation with monochloramine increase d by approximately 250 percent compared to the post-shoulder rate with mono chloramine alone. The level of synergy from using chlorine dioxide as the p rimary disinfectant was lower compared to that for ozone.